1. Field of the Invention
The present invention relates to annular-type blow out preventers, and more particularly to an improved annular blow out preventer arrangement with a body and head that are threadably engaged and a sealing arrangement prevents corrosive attack to the threads that join the head and body, an area typically attacked during use. Even more particularly, the present invention relates to an improved annular-type blow out preventer having a moving piston that forces a packing unit into close engagement with a drill pipe and wherein a method of preventing corrosive attack removes one or more threads from the threaded connection that joins the head to the body and replaces those threads with a one-way or lip-type annular seal.
2. General Background of the Invention
Blow out preventers have long been used in the oil and gas well drilling industry to prevent a catastrophic blow out condition. Such a blow out condition can occur when a well is not maintained by well drilling fluids in proper hydrostatic balance. Down hole well pressure can overcome the hydrostatic pressure of drilling mud or like drilling fluid within the well. In such a situation, a blow out preventer closes the well bore to contain pressure and prevent a "blow out".
One of the most common types of blow out preventers is an annular-type blow out preventer that surrounds the drill pipe with a housing. Such devices have been sold commercially since the 1950's. One manufacturer sells these "BOPs" under the mark Hydril.RTM.. In order to access the central working portion of the apparatus (e.g., piston and packing unit), an upper head threadably engages and attaches to a lower body.
Examples of early blowout preventer constructions can be seen in the Allen patents 2,752,119 and 2,912,214. Another prior art example is U.S. Pat. No. 3,272,222 also issued to H. Allen, entitled "Blowout Preventer".
U.S. Pat. No. 3,561,723, issued to Edward T. Cugini, discloses a stripping blowout preventer device for use in well drilling operations, particularly in offshore subsea well drilling, to prevent fluid escaping from the well in the presence or absence of a well tool such as a pipe string, while rotating or stationary, or during removal of the string from the well hole.
U.S. Pat. No. 3,614,111, issued to John Regan, shows a stationary blowout preventer having a balloon-type packing unit with a central opening therein, a retrievable packing insert positioned within said opening by the engagement of a lower ring with the preventer and releasably secured therein by hydraulically releasable dogs latchingly engaging a latching notch in an upper ring, whereby a central rubber portion of the packing insert sealingly engages the pipe tool therethrough when the packing unit is pressurized by fluid. The blowout preventer has a fluid accumulator which absorbs the surge pressure and excess fluid to maintain a constant pressure on the packing insert as a pipe tool joint is stripped therethrough to maintain the sealing engagement between the insert and the pipe.
U.S. Pat. No. 3,647,174, issued to Robert K. LeRouax, discloses a blowout preventer of the ram-type, each having a bonnet which is detachably connected to the preventer body, and which bonnet is movable away from the preventer body to expose the ram therewith for permitting removal of the ram from the preventer for replacement or repair, wherein the ram piston and cylinder which are operable for moving the ram to and from its open and closed positions when the bonnet is secured to the preventer body are also operable for moving the bonnet away from and towards the preventer body when the bonnet is disconnected from the body.
U.S. Pat. No. 3,741,296, issued to F. Murman et al., discloses a method and apparatus to enable well blowout preventer packer replacement at a sub-sea well head location.
U.S. Pat. No. 3,744,749, issued to Robert LeRouax, shows a blowout preventer having new and improved means for supporting and guiding each ram in its travel to and from the open and closed positions whereby more uniform support, anti-cocking, and anti-heeling of the ram during such travel are accomplished.
U.S. Pat. No. 4,007,905, issued to James D. Mott, discloses retrievable blowout preventer ram seals which are adapted to be removed and/or inserted with apparatus working on the inside of the well pipe.
Over a long period of time, the hydraulic fluid can contain small metallic shavings and other contaminants and/or corrosive material that are generated by cyclical movement of the piston. Such corrosives and contaminants can attack the threaded connection that joins the head to the body. Over time, this corrosive attack can be so severe that the head and body appear to be welded together. In such a situation, even very large hydraulic jacks can not break the threads loose and the entire apparatus is ruined. Annular-type blow out preventers are very expensive, costing, for example, between $50,000-$100,000. When the head becomes permanently attached to the body because of corrosive attack, the blow out preventer is not repairable and must be junked.